1. Field of the Invention
The invention relates generally to hybrid packaging of microwave integrated circuits (MIC) and more specifically to MIC housings incorporating hermetic glass seals and alumina substrates.
2. Description of the Prior Art
The most common MIC packages are gold-plated metal enclosures made from an iron-nickel-cobalt alloy whose chemical composition is controlled within narrow limits to assure precise uniform thermal expansion properties, e.g. Carpenter Kovar.RTM., referred to below simply as "Kovar." (See, Richardson, E. F., "Select Material to Balance Benefits for Packaged MICs," Microwaves & RF, July, 1989, pp. 87-95, paraphrased here; and see "Carpenter Controlled-Expansion Alloys," a sales brochure published by Carpenter Technology, Carpenter Steel Division, Reading, Pa.) Kovar alloy is manufactured by Carpenter Technology to meet the requirements of ASTM Specification F-15-61T (Alloy 2).
Kovar has several benefits as a MIC package material. Most notably, coaxial feedthroughs can be fired directly into a Kovar package using simple processes. Kovar can also be easily welded and plated. And, Kovar is especially compatible with borosilicate glasses for hermetic glass-to-metal seals. The expansion coefficients of the two are closely matched at temperatures less than the setting temperature of the glass. Other metals and other glasses are not so easily matched and most do not form as good a seal as Kovar with borosilicate glass.
MIC packages are also made of aluminum. Kovar and aluminum have widely varying properties and offer different sets of benefits and detriments, when used in MIC packaging. Aluminum compares favorably to Kovar in terms of density, thermal conductivity, machinability and raw material cost. But aluminum cannot have glass seals fired into it, and plating is complicated and problematic compared to Kovar. Aluminum's high rate of thermal expansion makes direct attachment of alumina ceramic substrates impossible.
MIC's for military applications are built in accordance with MIL-M-38510, Appendix G, and MIL-STD-883. Adhesive and polymeric materials are not permissible for use in hermetic sealing according to MIL-M-38510. Therefore, either glass or ceramic are required for feedthroughs as insulating material. Hermetic seals are 100% tested according to MIL-STD-883, Method 1014. The rigorous requirements of MIL-STD-883, Method 1014 demand that superior glass seals and welds exist to prevent unacceptable yield loss.
The need in electronics for glass-to-metal seals is described by Scott, in U.S. Pat. No. 2,065,404, issued Dec. 22, 1936. The basic technique of forming oxides on the surface of iron-nickel alloys that will readily fuse with glass, at ordinary glass blowing temperatures, to form a vacuum-tight seal is described by Scott. Since some carbon always remains in the metal alloy and can cause small glass bubbles to form in a glass seal, Scott suggests that the carbon may be removed by heating the metal in moist hydrogen for at least two hours at a temperature of 950.degree. C.
Vacuum-tight bonds between pre-oxidized Kovar and borosilicate glass are made by heating the joint to the point the glass bonds to the Kovar, which is about 1,000.degree. C. The setting point of glass is approximately the temperature that the glass becomes viscous enough to yield to applied loads. The expansion rates of Kovar and borosilicate glasses are not perfectly matched, but the residual stresses are considered in the prior art to be negligible.
Corning Glass Works (Corning, N.Y.) borosilicate glasses 7052 and 7070 are preferred for use in feedthroughs, because these glasses are rugged, resist corrosion, and have the relatively low dielectric constants of 4.9 and 4.1, respectively. For microwave packaging, a constant impedance, usually 50.OMEGA. for all feedthroughs, is desired for optimum electrical performance. A lower dielectric constant (.epsilon..sub.r) allows a smaller outside diameter (d.sub.o) for a given impedance (Z.sub.o) and center pin size (d.sub.i), in accordance with Formula I. A smaller diameter feedthrough allows the package height to be minimized, with a decrease in size and weight.